Drum-type steam generation systems generally comprise three major components: an evaporator, a superheater and an economizer. The different components are put together to meet the operating needs of the unit. Some drum-type steam generation systems may not have a superheater or may include additional components such as reheaters.
The FIG. 1 is a depiction of an exemplary prior art evaporator system 100 of a drum-type steam generator that comprises an evaporator 102 and a steam drum 104. The steam drum 104 is in fluid communication with the evaporator 102. The steam drum 104 is both downstream and upstream of the evaporator 102, i.e., they lie in a recycle loop. In the operation of the evaporator system 100 of the FIG. 1, when the load on the evaporator 102 changes more water is drawn from the steam drum 104. For example, if there is a need for a greater amount of steam than that which was previously desired, the water level in the steam drum 104 drops. Feed water is then introduced to the steam drum 104 to maintain the predetermined operating water levels.
The steam drum 104 is therefore sized based on the steam needs for the drum-type steam generator. However, when additional requirements such as the water hold time exceeds the normal steam drum 104 water storage level for a single drum, it is desirable to increase the size of the steam drum 104. The water hold time (also sometimes termed the “holdup time”) is based on the measured liquid volume between normal water level (NWL) and the lowest (also sometimes referred to as the “lo-lo”) water level trip. The lowest water trip level is the minimum level at which there will be no danger of overheating any part of the steam generator during operation. This lowest water level is generally about 30 centimeters (about 1 foot) above the bottom of the drum, but varies according to drum diameter.
The normal water level is set below the high water level, as needed for water level measurement accuracy, margin to control feedwater flow and steam purity. In general, the location of normal water level results in about 15 seconds to 30 seconds of water volume (depending upon the flow rate) between the normal water level and the water level trip. The volume of water contained in the drum at these different heights can be calculated using simple formulas for the area of a circular segment.
One manner of increasing the water hold time of a single steam drum is to increase the length and/or the diameter of the drum. However, this may not be a viable option where space availability is limited. The use of larger diameter drums increases shell wall thicknesses to accommodate internal pressures. Thicker walled vessels however generally use longer heat up times when compared with thinner walled vessels resulting slower transient during start-up and/or load changes.
It is therefore desirable to increase the water hold time of the steam drum without incurring additional costs associated with increasing space or with increasing the wall thickness.